docs: fix latest version number

docker: ensure cleaner contexts for Docker builds

`make docker` builds the context from a tarball. We don't use git
archive as we want to allow local modifications without committing them.
This is not unlike nix work.

For the other Docker files, use a tailored `.dockerignore`:

 - dev, ignore everything except bin/akvorado
 - ipinfo-geoipupdate, ignore everything except the update script
 - nix, let nix handle it from the Git repository

docs: prepare for release

docs: prepare for 2.0.1

build: fix tagging of multiple images in Docker

build: update Nix dependency hashes

docs: highlight again the necessity to recreate the Kafka container

docs: last beta!

docs: prepare for another beta

docs: prepare for another beta